U.S. patent number 9,685,662 [Application Number 14/353,364] was granted by the patent office on 2017-06-20 for electrode material, electrode material manufacturing method, electrode, and secondary battery.
This patent grant is currently assigned to Kobe Steel, Ltd.. The grantee listed for this patent is KOBE STEEL, LTD.. Invention is credited to Mamoru Hosokawa, Sho Katsura, Toshiki Sato, Satoru Takada.
United States Patent |
9,685,662 |
Katsura , et al. |
June 20, 2017 |
Electrode material, electrode material manufacturing method,
electrode, and secondary battery
Abstract
An electrode material which has excellent tab weldability,
realizes reduction of a contact resistance with an active material
layer, and has good adhesion with a conductive material disposed in
an island shape, is provided. An electrode material 1 includes a
substrate 1a including a metal foil and a conductive material 1b
containing carbon, wherein the conductive material 1b is disposed
in an island shape on the surface of the substrate 1a when observed
with a visual field of 300 .mu.m square, and the conductive
material is fixed to the surface of the substrate together with a
hydrophobic resin and a water-soluble resin.
Inventors: |
Katsura; Sho (Hyogo,
JP), Takada; Satoru (Hyogo, JP), Hosokawa;
Mamoru (Hyogo, JP), Sato; Toshiki (Hyogo,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
KOBE STEEL, LTD. |
Kobe-shi |
N/A |
JP |
|
|
Assignee: |
Kobe Steel, Ltd. (Kobe-shi,
JP)
|
Family
ID: |
48167746 |
Appl.
No.: |
14/353,364 |
Filed: |
October 22, 2012 |
PCT
Filed: |
October 22, 2012 |
PCT No.: |
PCT/JP2012/077217 |
371(c)(1),(2),(4) Date: |
April 22, 2014 |
PCT
Pub. No.: |
WO2013/061914 |
PCT
Pub. Date: |
May 02, 2013 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140287319 A1 |
Sep 25, 2014 |
|
Foreign Application Priority Data
|
|
|
|
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Oct 25, 2011 [JP] |
|
|
2011-234320 |
Sep 5, 2012 [JP] |
|
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2012-194663 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/663 (20130101); H01M 4/667 (20130101); H01M
4/661 (20130101); H01M 4/668 (20130101); H01M
10/0525 (20130101); Y02E 60/10 (20130101); H01M
4/139 (20130101); Y02P 70/50 (20151101); H01M
4/13 (20130101) |
Current International
Class: |
H01M
4/66 (20060101); H01M 4/13 (20100101); H01M
4/139 (20100101); H01M 10/0525 (20100101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
1 387423 |
|
Feb 2004 |
|
EP |
|
6-260164 |
|
Sep 1994 |
|
JP |
|
9-97625 |
|
Apr 1997 |
|
JP |
|
10-144298 |
|
May 1998 |
|
JP |
|
2001-351612 |
|
Dec 2001 |
|
JP |
|
2003-249223 |
|
Feb 2002 |
|
JP |
|
2002-352796 |
|
Dec 2002 |
|
JP |
|
2003-249223 |
|
Sep 2003 |
|
JP |
|
2006-4739 |
|
Jan 2006 |
|
JP |
|
2007-226969 |
|
Sep 2007 |
|
JP |
|
2009-252396 |
|
Oct 2009 |
|
JP |
|
2009-266466 |
|
Nov 2009 |
|
JP |
|
2010-86866 |
|
Apr 2010 |
|
JP |
|
2010-135338 |
|
Jun 2010 |
|
JP |
|
2011-49231 |
|
Mar 2011 |
|
JP |
|
2011-86636 |
|
Apr 2011 |
|
JP |
|
10-2011-0107401 |
|
Sep 2011 |
|
KR |
|
200913348 |
|
Mar 2009 |
|
TW |
|
WO 03/032418 |
|
Apr 2003 |
|
WO |
|
WO 2009/011249 |
|
Jan 2009 |
|
WO |
|
Other References
Machine translation of JP2003-249223, printed Aug. 7, 2016. cited
by examiner .
Extended European Search Report issued Jun. 17, 2015 in Patent
Application No. 12843029.5. cited by applicant .
International Search Report and Written Opinion issued Jan. 15,
2013 in PCT/JP2012/077217 with English Translation. cited by
applicant.
|
Primary Examiner: Laios; Maria J
Attorney, Agent or Firm: Oblon, McClelland, Maier &
Neustadt, L.L.P.
Claims
The invention claimed is:
1. An electrode material, comprising: a substrate comprising a
metal foil; and a conductive material placed on at least one
surface of the substrate, wherein the conductive material is
disposed in an island shape on the surface of the substrate when
observed with a visual field of 300 .mu.m .sup.2, the conductive
material is attached to the surface of the substrate with a
hydrophobic resin and a water-soluble resin, and an attached amount
of a sum of the conductive material, the water-soluble resin and
the hydrophobic resin per unit area on the surface of the substrate
ranges from 0.01 to 0.50 g/m.sup.2.
2. An electrode in a secondary battery, the electrode comprising:
the electrode material according to claim 1; and an active material
layer formed on a surface of the conductive material in the
electrode material.
3. A secondary battery, comprising a positive electrode, and a
negative electrode, wherein at least one of the positive electrode
and the negative electrode is the electrode according to claim
2.
4. The electrode material according to claim 1, wherein the
attached amount of the sum of the conductive material, the
water-soluble resin and the hydrophobic resin per unit area on the
surface of the substrate is 0.03 g/m.sup.2 or more.
5. The electrode material according to claim 1, wherein the
attached amount of the sum of the conductive material, the
water-soluble resin and the hydrophobic resin per unit area on the
surface of the substrate is 0.05 g/m.sup.2 or more.
6. The electrode material according to claim 1, wherein the
attached amount of the sum of the conductive material, the
water-soluble resin and the hydrophobic resin per unit area on the
surface of the substrate is 0.40 g/m.sup.2 or less.
7. The electrode material according to claim 1, wherein the
attached amount of the sum of the conductive material, the
water-soluble resin and the hydrophobic resin per unit area on the
surface of the substrate is 0.30 g/m.sup.2 or less.
8. The electrode material according to claim 1, wherein the
conductive material comprises a carbonaceous material.
9. The electrode material according to claim 1, wherein the
hydrophobic resin is at least one selected from the group
consisting of a polyethylene-based resin, a polypropylene resin, a
polyester resin, a urethane-based resin, and an acrylic resin.
10. The electrode material according to claim 1, wherein the
water-soluble resin is at least one selected from the group
consisting of a carboxyvinyl polymer, a carboxymethyl cellulose and
a salt thereof, and a polyvinyl alcohol.
11. A method for manufacturing the electrode material according to
claim 1, the method comprising applying a slurry comprising the
conductive material, the water-soluble resin, and an aqueous
emulsion solution of the hydrophobic resin on the surface of the
substrate, and subsequently drying the slurry, wherein the
conductive material is agglomerated in the slurry before or after
the slurry is applied on the substrate.
12. The method according to claim 11, wherein the conductive
material comprises carbon, and a content of the carbon in the
slurry is from 0.1 to 7% by mass.
13. The method according to claim 11, further comprising after the
drying, pressure-bonding or rolling the conductive material.
14. The method according to claim 11, wherein the slurry comprises
the water-soluble resin in an amount of 0.25% by mass or more and
the hydrophobic resin in an amount of 0.01% by mass or more, and a
sum of the amount of the water-soluble resin and the amount of the
hydrophobic resin is 11% by mass or less.
15. The method according to claim 11, wherein the conductive
material is agglomerated in the slurry before the slurry is applied
on the substrate.
16. The method according to claim 11, wherein the conductive
material is agglomerated in the slurry after the slurry is applied
on the substrate.
17. The method according to claim 11, wherein the carbon has an
average particle diameter of from 0.01 to 1 .mu.m.
Description
TECHNICAL FIELD
The present invention relates to an electrode material to be used
for an electrode in a secondary battery, a method for manufacturing
the same, an electrode using the electrode material, and a
secondary battery using the electrode.
BACKGROUND ART
Studies on collectors in which a carbonaceous conductive material
is applied on a metal foil such as an aluminum foil or a copper
foil to be used as a substrate of an electrode for a secondary
battery have been hitherto conducted in various research
institutes. Also, a large number of patent applications have been
filed and, for example, Patent Documents 1 to 4 are
exemplified.
Patent Documents 1 and 2 describe collectors in which a film
composed of carbon fine particles that are conductive materials and
a film-forming compound is formed on the surface of a substrate
such as an aluminum foil or a copper foil. Moreover, Patent
Document 3 describes a collector in which a conductive layer
composed of a carbon powder (conductive material) and a binder is
placed between active materials. Further, Patent Document 4
describes a collector in which a conductive coating layer
containing carbon as a conductive agent (conductive material) is
placed on the surface. In these, internal resistance of batteries
using these collectors is reduced by reducing contact resistance
between the collectors and active material layers formed thereon,
and thus it has been intended to improve high-speed
charge-discharge characteristics and cycle characteristics of the
batteries.
PRIOR ART DOCUMENTS
Patent Documents
Patent Document 1: JP-A-2007-226969
Patent Document 2: JP-A-2010-135338
Patent Document 3: JP-A-9-97625
Patent Document 4: JP-A-2001-351612
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
Here, the constitutions of the collectors in conventional
technologies described in Patent Documents 1 to 4 are collectively
explained with reference to FIG. 6. FIG. 6 is a schematic
cross-sectional view for illustrating the constitutions of the
collectors in the conventional technologies. As shown in FIG. 6, in
a collector 3 in the conventional technologies, a conductive
material layer 3b is homogeneously formed on the surface of a
substrate 3a composed of a metal foil. Namely, the whole surface of
the substrate 3a is covered with the conductive material layer
3b.
As shown in FIG. 6, when the conductive material layer 3b is
homogeneously formed on the surface of the substrate 3a, there is a
problem that the conductive material layer 3b hinders welding to
worsen weldability when a metal-made tab (not shown in the figure)
or the like for electrically connecting the collector 3 and a
terminal of a battery, for preparing the battery using the
collector 3, is welded on the surface.
In order to solve such a problem, the applicant of the present
application has invented an electrode material (collector) which is
constituted so that a conductive material is disposed in an island
shape on the surface of a substrate. Thereby, reduction of contact
resistance and weldability can be both achieved.
Here, for the following reason, it has been desired to further
improve adhesion between the conductive material disposed in the
island shape (hereinafter referred to as island structure as
required) on the substrate surface and the substrate. In the case
of preparing an electrode using the collector in which the
conductive material is disposed in an island shape on the substrate
surface, an active material layer is further applied and pressed on
the island structure of the conductive material, so that the
adhesion between the active material layer and the substrate is
secured. However, in the case where the adhesion between the island
structure of the conductive material and the substrate is weak, the
island structure of the conductive material tends to be easily
peeled by handling of the collector and contact of the surface of
the collector with a roller equipped on a coating machine for the
active material layer after the step of the formation of the island
structure of the conductive material and throughout the coating
step of the active material layer. Consequently, it is considered
that the effect of reducing the contact resistance is decreased by
the peeling of the island structure of the conductive material.
The present invention has been devised in view of the
above-described problem and an object of the present invention is
to provide: an electrode material which has excellent tab
weldability, realizes reduction of the contact resistance with the
active material layer, and has good adhesion between the conductive
material disposed in the island shape and the substrate; a method
for manufacturing the same; an electrode using the electrode
material; and a secondary battery which realizes reduction of
internal resistance by using the electrode.
Means for Solving the Problems
In order to solve the above problems, the electrode material
according to the present invention contains: a substrate including
a metal foil; and a conductive material placed on at least one
surface of the substrate, wherein the conductive material is
disposed in an island shape on the surface of the substrate when
observed with a visual field of 300 .mu.m square, and the
conductive material is fixed to the surface of the substrate
together with a hydrophobic resin and a water-soluble resin.
According to such a constitution, the surface of the substrate
including the metal foil with the conductive material such as
carbon has a portion which is not covered with the conductive
material. Preferably, by controlling the coverage with the
conductive material to 80% or less, the portion which is not
covered with the conductive material becomes 20% or more.
Therefore, in the case where the electrode material is used as a
collector for an electrode in a lithium ion secondary battery, it
becomes easy to weld a metal-made tab for connecting the collector
and a terminal of the battery. Moreover, by placing the conductive
material on the surface of the substrate so that the coverage
preferably becomes 1% or more, in the case where the electrode
material is used as a collector for an electrode in a lithium ion
secondary battery, the contact resistance between the collector and
the active material layer laminated on the collector is reduced.
Further, the adhesion between the conductive material and the
substrate is improved by the hydrophobic resin, and a coatability
of the substrate with a slurry containing the conductive material
on the substrate is improved by the water-soluble resin in the
manufacturing step. Thereby, the island structure is disposed in an
island structure as a mixture of the conductive material and these
resins with good adhesion to the substrate that is a metal
foil.
Furthermore, in the electrode material according to the present
invention, the attached amount of the sum of the conductive
material, the water-soluble resin, and the hydrophobic resin per
unit area on the surface of the substrate is preferably in the
range of 0.01 to 0.50 g/m.sup.2. Here, the conductive material may
be placed on one surface of the substrate or may be placed on both
surfaces of the substrate, but the range of the above attached
amount shows a preferable range on each surface on which the
conductive material is placed.
According to such a constitution, the conductive material is
disposed in the island shapes on the surface of the substrate by
controlling the attached amount to the above range and the coverage
is adjusted so that the substrate is appropriately exposed by
controlling the attached amount to 0.50 g/m.sup.2 or less, so that
the metal-made tab or the like is satisfactorily welded. Moreover,
by controlling the attached amount to 0.01 g/m.sup.2 or more, in
the case where the electrode material is used, for example, as a
collector for the electrode in a lithium ion secondary battery, the
contact resistance between the collector and the active material
layer laminated on the collector is reduced.
The method for manufacturing the electrode material according to
the present invention includes an application step of applying a
slurry containing the conductive material, the water-soluble resin,
and an aqueous emulsion solution of the hydrophobic resin on the
surface of the substrate, and a drying step of drying the slurry in
this order, wherein the conductive material is agglomerated in the
solution before or after the slurry is applied on the
substrate.
According to such a method, since an aqueous emulsion solution of
the hydrophobic resin is used, the hydrophobic resin is
homogeneously dispersed in the slurry. Moreover, since the
water-soluble resin is mixed into the slurry, in the application
step, wettability of the slurry containing the water-soluble resin
is improved and thus the slurry is satisfactorily applied without
being repelled on the surface of the substrate to form drops.
Further, since the slurry containing the conductive material to be
applied on the surface of the substrate is controlled so that the
conductive material is agglomerated before application or after
application, the conductive material in the slurry applied on the
surface of the substrate is disposed in the island shape on the
surface of the substrate. By drying the slurry in the drying step,
the conductive material is fixed to the surface of the substrate
together with the hydrophobic resin and the water-soluble
resin.
In the method for manufacturing the electrode material according to
the present invention, it is preferred that, in the application
step, the slurry containing carbon as the conductive material in an
amount of 0.1 to 7% by mass is applied on the surface of the
substrate.
According to such a method, the conductive material is
appropriately agglomerated in the slurry before or after the
application on the substrate and the conductive material is
disposed in the island shape on the surface of the substrate.
In the method for manufacturing the electrode material according to
the present invention, it is preferred that, in the application
step, the slurry contains the water-soluble resin in an amount of
0.25% by mass or more and the hydrophobic resin in an amount of
0.01% by mass or more, and a sum of the amount of the water-soluble
resin and the amount of the hydrophobic resin in the slurry is 11%
by mass or less.
According to such a method, the wettability of the slurry with
respect to the substrate is appropriately improved by the
water-soluble resin having the content in this range and the slurry
is satisfactorily applied on the substrate. Moreover, the island
structure is fixed to the substrate with good adhesion by the
hydrophobic resin having the content in this range. Further, the
effect of reducing the contact resistance can be obtained by
controlling the content of all the resins to 11% by mass or
less.
The electrode in a secondary battery according to the present
invention is an electrode in a secondary battery using the
above-described electrode material, wherein an active material
layer is formed on a surface of the conductive material in the
electrode material.
According to such a constitution, the contact resistance between
the electrode material that is a collector and the active material
layer is reduced by the conductive material disposed in the island
shape on the surface of the substrate.
The secondary battery according to the present invention includes a
positive electrode and a negative electrode, wherein at least one
of the positive electrode and the negative electrode is the
electrode according to the present invention.
According to such a constitution, since the contact resistance
between the electrode material that is a collector and the active
material layer is reduced in the electrode in the secondary
battery, the internal resistance is reduced in the secondary
battery.
Advantages of the Invention
According to the electrode material of the present invention, a
conductive material is disposed in the island shape on the surface
of a metal foil that is a substrate and the attached amount of the
conductive material is limited, and thus, the contact resistance
between the active material layer and the electrode material can be
reduced while weldability of a tab or the like is secured.
Moreover, the island structure is adhered to the metal foil that is
a substrate using the hydrophobic resin and water-soluble resin as
binders and thus peeling of the formed island structure can be
suppressed during the handling of the electrode material and in the
manufacturing step before coating with the active material layer,
and the effect of reducing the contact resistance by the island
structure is not impaired.
According to the method for manufacturing the electrode material of
the present invention, since the island structure of the conductive
material can be formed on the surface of the substrate with good
adhesion, an electrode material having a good weldability of a tab
or the like and capable of reducing the contact resistance with the
active material layer, can be manufactured with stable quality.
Moreover, owing to the use of an aqueous solution, an environmental
load induced by solvent vapor in the manufacturing step can be
reduced.
Further, according to the method for manufacturing the electrode
material of the present invention, since the carbon is used in an
appropriate concentration as a conductive material, an electrode
material in which a suitable island structure is formed can be
manufactured.
Furthermore, according to the method for manufacturing the
electrode material of the present invention, since the resins are
used in appropriate concentration, an electrode material having
good quality can be manufactured.
According to the electrode of the present invention, the contact
resistance between the electrode material and the active material
layer can be reduced.
Moreover, according to the secondary battery of the present
invention, internal resistance can be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view for illustrating the
structure of the collector according to the present invention.
FIG. 2 is a schematic cross-sectional view for illustrating the
structure of an electrode using the collector according to the
present invention.
FIG. 3 is a schematic cross-sectional view for illustrating the
structure of a secondary battery using the electrode according to
the present invention.
FIG. 4 is a flow chart showing a flow of the method for
manufacturing a collector according to the present invention.
FIG. 5 is a picture of the surface of a sample of the collector
according to Example on a scanning electron microscope.
FIG. 6 is a schematic cross-sectional view for illustrating the
structure of collectors according to conventional technologies.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
The following will describe an embodiment of the electrode material
(hereinafter also referred to as collector) according to the
present invention in detail.
[Structure of Collector]
The structure of the collector according to the present embodiment
will be described with reference to FIG. 1.
The collector (electrode material) 1 according to the present
embodiment is composed of a substrate 1a composed of a metal foil
and a conductive material 1b disposed in the island shape on the
surface of the substrate 1a. Moreover, the conductive material 1b
is disposed on both surfaces of the substrate 1a. The conductive
material 1b may be disposed on one surface of the substrate 1a.
In the present specification, the "island shape" means a state
where the conductive material 1b is disposed so that at least a
part of the surface of the substrate 1a is exposed without being
covered with the conductive material 1a. For example, a plurality
of agglomerates of the conductive material 1b may be disposed with
being isolated one another as shown in FIG. 1 or the agglomerates
may be joined one another to be disposed in a reticulate form.
The collector 1 according to the present embodiment can be suitably
used, for example, as a collector for an electrode in a lithium ion
secondary battery. The electrode using the collector 1 will be
described later.
By controlling the collector 1 so as to have a structure in which
the conductive material 1b is disposed in the island shape on the
surface of the substrate 1a when observed with a visual field of
300 .mu.m square (i.e., a square visual field having an area of
about 0.1 mm.sup.2) in an arbitrary region of the surface of the
substrate 1a on which the conductive material 1b is disposed, in
the case where the collector 1 is used as an electrode in the
battery, tab weldability can be improved during welding a
metal-made tab (not shown in the figure) that electrically connects
the collector 1 with a terminal of the battery.
Moreover, the coverage of the surface of the substrate 1a with the
conductive material 1b in the substrate 1a is preferably from 1 to
80% when observed with a visual field of 300 .mu.m square in an
arbitrary region of the surface of the substrate 1a on which the
conductive material 1b is disposed. In the case where the coverage
with the conductive material 1b is 1% or more, the contact
resistance between the collector 1 and the active material layer 2
(see FIG. 2) to be laminated on the surface of the collector 1 in
the case of using as an electrode in the secondary battery, can be
further reduced. Further, by controlling the coverage of the
surface of the substrate 1a with the conductive material 1b to 80%
or less, further satisfactory tab weldability can be secured. The
above coverage is more preferably from 10 to 70%, and further
preferably from 30 to 60%.
The conductive material 1b is preferably disposed uniformly on the
surface of the substrate 1a per at least a unit of the observed
area. Since the conductive material 1b is disposed so as to be in
the island shape within the region having a sufficiently small
area, practically uniform contact resistance and tab weldability
are obtained.
Here, the coverage of the surface of the substrate 1a with the
conductive material 1b can be determined by photographing the
surface of a prepared sample using SEM (scanning electron
microscope) and calculating a covered area of the substrate surface
with carbon included in the photographed visual field by image
processing.
Moreover, the attached amount of the conductive material 1b per
unit area on the surface of the substrate 1a is preferably in the
range of 0.01 to 0.50 g/m.sup.2 as the attached amount of the sum
of the conductive material 1b and the water-soluble resin and
hydrophobic resin that are binders and are not shown in the figure.
Also, the attached amount is more preferably 0.03 g/m.sup.2 or
more, and further preferably 0.05 g/m.sup.2 or more. The attached
amount is more preferably 0.40 g/m.sup.2 or less, and further
preferably 0.30 g/m.sup.2 or less. In the case where the conductive
material 1b is placed on both surfaces of the substrate 1a, it is
preferred to control the attached amount to the range on each
surface.
By controlling the attached amount to the range, the conductive
material 1b is disposed in the island shape on the surface of the
substrate 1a. Moreover, by controlling the attached amount to
preferably 0.50 g/m.sup.2 or less, more preferably 0.40 g/m.sup.2
or less, and further preferably 0.30 g/m.sup.2 or less, the
coverage becomes such one that the surface of the substrate 1a is
appropriately exposed, so that good tab weldability is obtained.
Further, by controlling the attached amount to preferably 0.01
g/m.sup.2 or more, more preferably 0.03 g/m.sup.2 or more, and
further preferably 0.05 g/m.sup.2 or more, the contact resistance
between the collector 1 and the active material layer 2 (see FIG.
2) to be laminated on the surface of the collector 1 in the case of
using as an electrode in the secondary battery, can be further
reduced.
Here, the attached amount of the conductive material 1b per unit
area on the surface of the substrate 1a can be determined as
follows. First, the mass of a sample obtained by applying a
solution containing the conductive material 1b, the water-soluble
resin, and the hydrophobic resin on the substrate 1a and drying the
solution, is measured. Then, the sample surface is wiped with water
and alcohol and the mass thereof after removal of the conductive
material 1b, the water-soluble resin, and the hydrophobic resin, is
measured. By dividing mass difference between them by the area of
the sample, the attached amount of the conductive material 1b per
unit area on the surface of the substrate 1a can be calculated.
(Substrate)
As the substrate 1a, a metal such as aluminum (Al) or copper (Cu)
that is commonly used as an electrode material for secondary
batteries, can be used. In the case of using as an electrode
material for secondary batteries, the substrate 1a is generally
used in a form of a foil having a thickness of about 5 to 50 .mu.m.
In the method for manufacturing the collector according to the
present embodiment, in the case where a rolling step is carried
out, rolling may be performed to achieve thinning after the
solution containing the conductive material 1b is applied on the
plate-shaped or thick foil-shaped substrate 1a and dried.
The substrate 1a is not limited to Al, Cu and the like having a
specific composition and, in the case of using as an electrode,
various pure metals and alloys thereof suitable for the use
environment of the electrode can be used.
(Conductive Material)
The conductive material 1b is disposed in the island shape so that
it covers 1 to 80% of the surface of the substrate 1a when observed
with a visual field of 300 .mu.m square and reduces the contact
resistance between the collector 1, which is constituted of the
conductive material together with the substrate 1a, and the active
material layer (see FIG. 2).
As the conductive material 1b, a carbonaceous conductive material
(carbon) can be used. As the carbonaceous conductive material,
natural or artificial crystalline graphite, expanded graphite,
artificial graphite, pyrolytic graphite, or various carbon blacks
such as acetylene black, can be used.
Moreover, with regard to the island structure composed of the
conductive material 1b, good adhesion to the surface of the
substrate 1a is secured by a binder (not shown in the figure) to be
mentioned later.
(Binder)
The binder (not shown in the figure) is constituted by a
hydrophobic resin for improving adhesion between the island
structure composed of the conductive material 1b and the surface of
the substrate 1a and a water-soluble resin for improving
applicability when a solution containing the hydrophobic resin and
the conductive material 1b is applied on the surface of the
substrate 1a. Thereby, the adhesion between the substrate 1a and
the island structure of the conductive material 1b is enhanced and
the island structure is formed on the surface of the substrate 1a
with uniform distribution.
Since the hydrophobic resin is necessarily mixed with the
water-soluble resin, the hydrophobic resin is preferably a resin
capable of forming an aqueous emulsion. As such a hydrophobic
resin, for example, one kind or two or more kinds as a mixture
selecting from polyethylene-based resins, polypropylene resins,
polyester resins, urethane-based resins, acrylic resins and the
like, can be used.
Moreover, as the water-soluble resin, for example, one kind or two
or more kinds as a mixture selecting from carboxyvinyl polymers,
carboxymethyl cellulose and salts thereof (sodium salt, ammonium
salt), polyvinyl alcohol and the like, can be used.
(Electrode)
Next, the constitution of the electrode in a lithium ion secondary
battery using the collector 1 according to the present embodiment
is explained with reference to FIG. 2.
An electrode 10 shown in FIG. 2 is constituted by the collector 1
according to the present embodiment and the active material layer 2
laminated on the surfaces (both surfaces) of the collector 1. In
the case of constituting the positive electrode in the lithium ion
secondary battery, a metal such as Al or an Al alloy can be used as
the substrate of the collector 1. Moreover, as a positive electrode
active material, conventional materials, for example,
lithium-containing oxides such as LiCoO.sub.2, LiNiO.sub.2, and
LiMn.sub.2O.sub.4 can be used. A method for manufacturing the
active material layer 2 in the positive electrode is also not
particularly limited and the layer can be manufactured by
conventional methods, for example, by adding a binder, and if
needed, a conductive material, a solvent and the like, to the
powdery lithium-containing oxide described above and thoroughly
kneading them, and subsequently applying the kneaded article on the
collector 1, followed by drying and pressing. The active material
layer 2 may be laminated on one surface on which the conductive
material 1b has been placed.
Moreover, in the case of constituting the negative electrode in the
lithium ion secondary battery, as the substrate of the collector 1,
a metal such as Cu, a Cu alloy, nickel (Ni), an Ni alloy or
stainless steel, can be used. Further, as a negative electrode
active material, for example, a graphite-based carbon material can
be used, and production can be made in a similar manner as in the
method for manufacturing the active material layer 2 in the
positive electrode.
(Secondary Battery)
The following will describe the constitution of the lithium ion
secondary battery using the electrode 10 using the collector 1
according to the present embodiment, with reference to FIG. 3 (see
FIG. 2 as required).
The lithium ion secondary battery (secondary battery) 20 shown in
FIG. 3 includes a positive electrode 11 and a negative electrode 12
that are the electrode 10 using the collector 1 according to the
present embodiment, a separator 13, and an electrolytic solution
14. The positive electrode 11 and the negative electrode 12 are
separated by the separator 13, and the electrolytic solution 14
fills the space between the positive electrode 11 and the negative
electrode 12 and the separator 13. Moreover, the entire lithium ion
secondary battery 20 is housed in a vessel (not shown in the
figure), and a metal-made tab (not shown in the figure) is welded
to each of the positive electrode 11 and the negative electrode 12,
which are electrically connected to an electrode terminal (not
shown in the figure).
In the positive electrode 11 and the negative electrode 12,
respective active material layers 2 containing each of the
above-described positive electrode active material and negative
electrode active material are formed on the surface of the
collector 1 according to the present embodiment.
Moreover, the separator 13 and the electrolytic solution 14 can be
each constituted using conventional materials. As the separator 13,
for example, a polyethylene-based microporous film having a
thickness of 20 to 30 .mu.m can be used. As the electrolytic
solution 14, for example, a non-aqueous electrolytic solution
obtained by dissolving an electrolyte such as LiPF.sub.6 or
LiBF.sub.4 in an organic solvent such as propylene carbonate or
ethylene carbonate, can be used.
The following will describe the method for manufacturing the
collector 1 according to the present embodiment, with reference to
FIG. 4 (see FIG. 1 as required).
[Manufacturing Method]
As shown in FIG. 4, the method for manufacturing the collector 1 in
the present embodiment includes an application step S1 and a drying
step S2 in this order.
The collector 1 can be prepared by a manufacturing method including
the application step S1 of applying a slurry containing the
conductive material 1b, an aqueous emulsion of the hydrophobic
resin and the water-soluble resin on the surface of the substrate
1a composed of a metal foil and agglomerating the conductive
material 1b in the slurry before or after the application on the
substrate 1a, and the drying step S2 of drying the slurry.
Here, the conductive material 1b homogeneously dispersed
immediately after the slurry is formulated is agglomerated with the
passage of time. Namely, the conductive material 1b is agglomerated
before or after the application of the formulated slurry on the
surface of the substrate 1a. In the slurry applied on the surface
of the substrate 1a, the conductive material 1b form a state of
being agglomerated in the island shape. Thereafter, the slurry is
dried to fix the conductive material 1b in the island shape on the
surface of the substrate 1a. On this occasion, the hydrophobic
resin fills the space between the substrate 1a, and the particles
of the conductive material 1b being in contact with the substrate
1a and the island structure of the conductive material 1b can be
strongly fixed to the surface of the substrate 1a with good
adhesion. Namely, the island structure is fixed to the surface of
the substrate 1a as a mixed body of the conductive material 1b, the
hydrophobic resin, and the water-soluble resin to be described
later.
The following will describe the application step in detail.
(Application Step)
First, the application step S1 will be described.
In order to agglomerate the conductive material 1b so as to be an
ideal island structure, it is effective to control the particle
diameter and concentration of the conductive material 1b and the
concentration of the hydrophobic resin and the water-soluble resin
as binders, which are contained in the slurry. In the case where
the particle diameter and concentration of the conductive material
1b and the concentration of the binders are controlled, viscosity
of the slurry is varied, so that coatability and distribution of
the conductive material 1b after drying are changed. In the case of
using carbon as the conductive material 1b, it is preferred to use
carbon having an average particle diameter of 0.01 to 1 .mu.m.
Also, the state where carbon is contained in an amount of 0.1 to 7%
by mass in the slurry is preferred. By controlling the average
particle diameter to 0.01 .mu.m or more and the concentration
(content) to 7% by mass or less, the viscosity of the slurry is not
exceedingly increased and the agglomeration of carbon particles one
another becomes not exceedingly large, and thus, the ideal island
structure can be obtained. Moreover, by controlling the average
particle diameter of the carbon as the conductive material 1b to 1
.mu.m or less and the concentration of the carbon to 0.1% by mass
or more, the island structure is formed and also contact points
between the carbon as the conductive material 1b and the metal foil
as the substrate 1a sufficiently exist, so that the effect of
reducing the contact resistance between the collector 1 and the
active material layer 2 (see FIG. 2) can be obtained.
Also, by using the hydrophobic resin, the island structure of the
conductive material 1b can be disposed on the substrate 1a that is
a metal foil with good adhesion. Therefore, when the electrode is
prepared, peeling of the island structure of the conductive
material 1b can be prevented until the application step of the
active material layer 2 (see FIG. 2).
The concentration of the hydrophobic resin is preferably 0.01% by
mass or more and about 11% by mass or less, more preferably 0.10%
by mass or more, and further preferably 0.25% by mass or more.
Furthermore, by dispersing the hydrophobic resin in water as an
aqueous emulsion, the hydrophobic resin and the water-soluble resin
can be mixed.
As the hydrophobic resin (aqueous emulsion resin), for example, one
kind or two or more kinds as a mixture selecting from
polyethylene-based resins, polypropylene resins, polyester resins,
urethane-based resins, acrylic resins and the like, can be
used.
The slurry that is an aqueous emulsion solution of the hydrophobic
resin in which the conductive material 1b is dispersed is repelled
in the above-described concentration range of the hydrophobic resin
owing to water repellency of the surface of the substrate 1a, and
hence there is such a problem in coatability that the slurry
becomes in a so-called water-repelled state to form drops. In order
to perform coating so that the slurry is not repelled on the
surface of the substrate 1a and does not form drops, it is
necessary to increase the concentration of the hydrophobic resin
exceeding the above-described concentration range to enhance the
viscosity of the slurry sufficiently. However, in the case where
the concentration of the hydrophobic resin is too high, the contact
resistance (i.e., internal resistance of the secondary battery)
becomes high, so that characteristics on the contact resistance
cannot be satisfied.
Accordingly, in the present embodiment, wettability with respect to
the substrate 1a that is a metal foil is improved by mixing the
water-soluble resin into an aqueous emulsion solution of the
hydrophobic resin. Thereby, coating of the substrate 1a with the
coating solution becomes possible without forming drops, and the
island structure of the conductive material 1b can be fixed to the
substrate 1a with good adhesion.
As the water-soluble resin, for example, one kind or two or more
kinds as a mixture selecting from carboxyvinyl polymers,
carboxymethyl cellulose and salts thereof (sodium salt, ammonium
salt), polyvinyl alcohol and the like, can be used.
The concentration of the water-soluble resin is preferably
controlled to 0.25% by mass or more and about 11% by mass or
less.
Furthermore, as an amount of resins necessary and sufficient for
satisfactorily adhering the island structure of the conductive
material 1b to the surface of the substrate 1a, total resin
concentration of the hydrophobic resin and the water-soluble resin
is preferably controlled to 0.5 to 11% by mass. Good coatability
and adhesion can be obtained by controlling the amount to 0.5% by
mass or more and a good effect of reducing the contact resistance
(internal resistance of the secondary battery) can be obtained by
controlling the amount to 11% by mass or less.
Moreover, as the conductive material 1b, a carbonaceous material
can be used. Specifically, natural or artificial crystalline
graphite, expanded graphite, artificial graphite, pyrolytic
graphite, and various carbon blacks such as acetylene black, can be
used.
For the application of the solution containing the conductive
material 1b on the surface of the substrate 1a, application methods
by means of various coaters such as a bar coater, a roll coater, a
gravure coater, a reverse gravure coater, a dip coater, and a spray
coater, can be used. The conductive material 1b is applied on both
surfaces or one surface of the substrate 1a.
(Drying Step)
The drying step S2 is a step of vaporizing the solvent after the
application step S1. In the drying step S2, drying may be performed
at room temperature or drying under heating may be performed by
using a heat treatment furnace or the like according to needs.
Moreover, drying temperature is preferably controlled to
100.degree. C. to 170.degree. C., and more preferably 140.degree.
C. to 170.degree. C. as a temperature at which water as a solvent
of the aqueous emulsion solution is easily evaporated (reason for
lower limit) and strength of the metal foil as the substrate 1a is
not lowered (reason for upper limit). Particularly, by controlling
the temperature to 140.degree. C. or higher, the hydrophobic resin
is softened by heat and thus the adhesion between the substrate 1a
and the hydrophobic resin is improved. As a result, the adhesion
between the substrate 1a and the conductive material 1b can be
further improved.
Moreover, drying time is not particularly limited but, in the case
where drying is conducted at 100.degree. C. to 170.degree. C., the
time is preferably controlled to 0.1 minutes to 5 minutes. The
solvent can be sufficiently evaporated by controlling the time to
0.1 minutes or more and a decrease in strength of the metal foil as
the substrate 1a can be prevented by controlling the time to 5
minutes or less.
(Pressure-Bonding/Rolling Step)
In order to further improve the adhesion and contact resistance,
pressure-bonding or rolling may be performed according to needs
after the drying step S2. As methods of pressure-bonding and
rolling, various rolling mills and roll press mills can be
used.
EXAMPLES
The following will describe the collector according to the present
embodiment by comparing Examples that satisfy the requirements of
the present invention with Comparative Examples that do not satisfy
the requirements of the present invention.
Samples were prepared by the following methods.
(Substrate)
As a substrate, an Al foil having a thickness of 15 .mu.m, which
was made of an Al alloy of 1000 type, or a Cu foil having a
thickness of 20 .mu.m, which was made of pure copper of 99.99%, was
used.
In Tables 1 to 3 described later, which of the Al foil or the Cu
foil was used in each sample was shown in the column of substrate
as "Al" or "Cu", respectively.
(Hydrophobic Resin)
As a hydrophobic resin, an aqueous emulsion solution using
polypropylene (PP) or polyethylene (PE) was prepared and the
prepared aqueous emulsion solution was diluted with pure water to
use. For comparison, a sample using no hydrophobic resin was also
prepared.
(Water-Soluble Resin)
As a water-soluble resin, carboxymethyl cellulose (CMC) sodium salt
was used. For comparison, a sample using no water-soluble resin was
also prepared.
(Conductive Material)
As a conductive material, carbon black (TOKABLACK #4300
manufactured by Tokai Carbon Co., Ltd.) having an average particle
diameter of 0.06 .mu.m was used.
(Application Step)
In the application, pure water was used as a solvent, and for each
sample, a solution (slurry) prepared at the concentration of the
carbon black (conductive material), the kind and concentration of
the hydrophobic resin, and the concentration of the water-soluble
resin, shown in Tables 1 to 3, was applied on the surface of the
substrate by means of a bar coater.
For the sample for evaluation of weldability, the above slurry
containing the carbon black was applied on both surfaces of the
substrate and, for the samples for evaluation of other items, the
slurry was applied on one surface of the substrate.
Moreover, for samples No. 1 to No. 11, for applying the solution
containing the conductive material, a count No. 2 bar coater was
used. Further, for samples No. 12 to No. 23, besides changing the
slurry composition, the attached amount of the conductive material
per unit area on the substrate was varied by appropriately changing
the count number of the bar coater used. These samples were
prepared in the same manner as in the cases of the other samples
except that the count number (count No. 2 for samples No. 12, No.
13 and No. 21, count No. 3 for samples No. 14 and No. 18 to No. 20,
count No. 5 for samples No. 15 to No. 17, and count No. 10 for
samples No. 22 and No. 23 were used) of the bar coater used in the
application step was different.
(Drying Step)
After the slurry containing the carbon black was applied on the
surface of the substrate, drying was performed by keeping in an
oven at 150.degree. C. for 1 minute.
<Evaluation Methods>
(Evaluation of Coatability)
For the evaluation of coatability, in the application step
described above, the case where the coated slurry formed drops on
the substrate owing to water repellency was ranked as bad (x) and
the case where no water repellency was observed and the coated
slurry could homogeneously cover the surface of the substrate was
ranked as good (.largecircle.).
(Evaluation of Adhesion)
A roller (having a diameter of 90 mm, a width of 50 mm and mass of
700 g) to which a pressure-sensitive adhesive tape having a
pressure-sensitive adhesive force of 2 N/40 mm, a width of 40 mm, a
length of 120 mm, and a thickness of 0.08 mm had been attached was
rolled on the surface of a sample by 100 mm and area of peeled
carbon black was estimated. The case where no peeling of carbon
black was observed was ranked as excellent (.largecircle.), the
case where 20% or less of peeling was observed was ranked as good
(.DELTA.), and the case where farther peeling was observed was
ranked as bad (x).
(Evaluation of Coverage)
The coverage with the conductive material (carbon) was determined
by photographing the surface of a sample at a magnification of 300
using a field emission-type scanning electron microscope (FE-SEM)
SU-70 manufactured by Hitachi Ltd. and calculating an area of the
substrate surface covered with carbon included in the photographed
visual field by image processing.
(Evaluation of Weldability 1)
For the evaluation of weldability in the case of using the Al foil
as a substrate, ten sheets of a sample in which the carbon black
(conductive material) had been formed on both surfaces of the Al
foil having a thickness of 15 .mu.m were overlaid, an aluminum foil
having a thickness of 30 .mu.m and an aluminum sheet having a
thickness of 250 .mu.m were placed at upper and lower parts thereof
(both ends), and welding was conducted in a condition of imparting
a constant pressure. The case where eight sheets or more were
welded was ranked as good and the case where only seven sheets or
less were welded was ranked as bad. For the welding, an ultrasonic
welding machine MH2026/CLF2500 manufactured by SONOBOND Co. was
used and the welding was performed at a weld time of 70 .mu.sec
under the conditions of a pressure of 0.28 MPa, an output of 400 W,
and an energy of 20 J.
(Evaluation of Weldability 2)
For the evaluation of weldability in the case of using the Cu foil
as a substrate, ten sheets of a sample in which the carbon black
(conductive material) had been formed on both surfaces of the Cu
foil having a thickness of 20 .mu.m were overlaid, and welding was
conducted in a condition of imparting a constant pressure. The case
where eight sheets or more were welded was ranked as good and the
case where only seven sheets or less were welded was ranked as bad.
For the welding, a spot welding machine HSW-02A manufactured by
Yokodai. jp Co. was used and the welding was performed at a weld
time of 500 .mu.sec at a voltage of 25V.
(Evaluation of Internal Resistance of Battery)
Both surfaces of a sample on which the carbon black (conductive
material) had been formed was coated at a thickness of 25 .mu.m per
one surface with a slurry obtained by mixing lithium cobaltate,
acetylene black and PVdF (polyvinylidene fluoride) to prepare a
positive electrode. Moreover, as a negative electrode, one obtained
by coating both surfaces of the Al foil with graphite at a
thickness of 35 .mu.m per one surface was used.
Using these positive and negative electrodes, a battery cell was
prepared using an HS flat cell manufactured by Hohsen Corp. For the
battery cell, after a three-cycle conditioning charge-discharge
treatment was conducted at a current of a charge-discharge rate of
0.2 C, a discharge test was carried out at plural current values of
0.2 C to 10 C. In the discharge curve at each current value
obtained from the discharge test, a relationship between the
current value and voltage value at discharging a capacity of 1 mAh
was plotted, and internal resistance was calculated based on the
slope of the straight line obtained by the plotting.
Moreover, using only a substrate of the Al foil having a thickness
of 15 .mu.m and having no conductive material as a collector, a
positive electrode was prepared in the same manner as in the case
of the other samples, and a battery cell was similarly prepared
using the positive electrode. For the battery cell, a discharge
curve was determined similarly to the battery cells using the other
samples, and internal resistance was calculated. Then, it was
judged that those exhibiting reduced internal resistance in
comparison to the internal resistance of the battery cell prepared
using the collector composed of only the substrate had an effect of
reducing the internal resistance. The internal resistance of the
battery cell prepared using only the Al foil that is a substrate as
the collector was 45 .OMEGA..
(Evaluation of Contact Resistance)
The contact resistance (effect of reducing resistance) in the case
of using the Cu foil as a substrate was measured as follows.
Both surfaces of a sample was sandwiched with two sheets of a
carbon cloth, the outsides were further sandwiched with two sheet
of a copper electrode having a contact area of 1 cm.sup.2, and
pressurization was conducted by imparting a load of 1 kgf (9.8 N)
to the copper electrode. Then, a current of 7.4 mA was applied
using a direct current power source and a voltage imparted between
the carbon cloths was measured with a voltmeter. The contact
resistance was determined by calculation from the above current
value, contact area, and measured voltage. Similar measurement was
performed using the substrate alone and it was judged that those
exhibiting reduced contact resistance in comparison to the case of
only the substrate had an effect of reducing the contact
resistance. The contact resistance in the case of only the Cu foil
as a substrate, which was not subjected to any surface treatment,
was about 100 [m.OMEGA.cm.sup.2 (milliohm square centimeter)].
(Evaluation of Attached Amount)
The attached amount of the conductive material per unit area on the
substrate was measured by the following procedure. In the present
Examples, the attached amount of the conductive material per unit
area on the substrate is an attached amount per unit area on one
surface of the substrate and is an attached amount of the sum of
the carbon black as a conductive material and the water-soluble
resin and hydrophobic resin as binders.
First, the mass of a sample obtained by applying a solution
containing the conductive material, the water-soluble resin and the
hydrophobic resin on the substrate and drying it was measured.
Then, the surface of the sample was wiped with water and dried.
Subsequently, the same surface of the sample was wiped with alcohol
to remove the hydrophilic resin, the lipophilic resin, and the
conductive material attached on the foil. The mass of the sample
after the removal of the resins and the conductive material was
measured. The attached amount of the conductive material per unit
area on the substrate was calculated by dividing the mass
difference of the sample before the removal of the conductive
material and after the removal of the conductive material by the
area of the substrate.
In the Examples, an Al foil or Cu foil having a size of 50 mm
square (i.e., area of 2,500 mm.sup.2) was used as the substrate
(i.e., sample).
Tables 1 and 2 show a list of characteristic evaluation results and
judgment results of samples prepared using the Al foil as a
substrate.
In Tables 1 and 2, ".largecircle." represents a good judgment
result and "x" represents a bad judgment result. Moreover, in
Tables 1 and 2, a numerical value judged as bad was underlined.
TABLE-US-00001 TABLE 1 Concentration Concentration Concentration of
of water- Total of carbon Species of hydrophobic soluble
Concentration Sample black hydrophobic resin resin of resins No.
Substrate (% by mass) resin (% by mass) (% by mass) (% by mass) 1
Al 1 PP 1 1 2 2 Al 0.3 PP 0.25 0.5 0.75 3 Al 6 PP 1 1 2 4 Al 3 PP 2
1 3 5 Al 2 PP 1 1 2 6 Al 5 PP 5 5 10 7 Al 1 PE 0.5 2 2.5 8 Al 0 --
0 0 0 9 Al 14 PP 5 5 10 10 Al 2 PP 0 2 2 11 Al 2 PP 1.5 0 1.5
Internal Weldability Weldability Sample resistance (1) (2) Coverage
No. Coatability Adhesion (.OMEGA.) (sheet) (sheet) (%) Judgment 1
.largecircle. .largecircle. 30 10 -- 24 .largecircle. 2
.largecircle. .DELTA. 41 10 -- 4 .largecircle. 3 .largecircle.
.largecircle. 36 8 -- 70 .largecircle. 4 .largecircle.
.largecircle. 28 10 -- 50 .largecircle. 5 .largecircle.
.largecircle. 29 10 -- 39 .largecircle. 6 .largecircle.
.largecircle. 30 8 -- 66 .largecircle. 7 .largecircle.
.largecircle. 38 10 -- 19 .largecircle. 8 -- -- 45 10 -- 0 X 9
.largecircle. .largecircle. 35 0 -- 100 X 10 .largecircle. X 27 10
-- 36 X 11 X -- -- -- -- -- X
TABLE-US-00002 TABLE 2 Concentration Concentration Concentration of
of water- Total of Species of hydrophobic soluble Concentration
Sample carbon black hydrophobic resin resin of resins No. Substrate
(% by mass) resin (% by mass) (% by mass) (% by mass) 12 Al 0.1 PP
0.25 0.75 1 13 Al 0.5 PP 0.1 0.5 0.6 14 Al 1 PP 0.25 0.5 0.75 15 Al
3 PP 0.01 0.5 0.51 16 Al 3 PP 0.05 0.75 0.8 17 Al 3 PP 0.1 1 1.1 18
Al 2 PE 0.5 2 2.5 19 Al 1 PP 1 1 2 20 Al 5 PP 2 2 4 21 Al 3 PP 5 5
10 22 Al 6 PP 1 1 2 23 Al 10 PP 5 5 10 Attached Internal
Weldability Weldability Sample amount resistance (1) (2) Coverage
No. (g/m.sup.2) Coatability Adhesion (.OMEGA.) (sheet) (sheet) (%)
Judgmen- t 12 0.01 .largecircle. .largecircle. 40 10 -- 4
.largecircle. 13 0.03 .largecircle. .largecircle. 39 10 -- 5
.largecircle. 14 0.05 .largecircle. .DELTA. 35 10 -- 7
.largecircle. 15 0.18 .largecircle. .largecircle. 30 10 -- 18
.largecircle. 16 0.23 .largecircle. .largecircle. 25 10 -- 28
.largecircle. 17 0.25 .largecircle. .largecircle. 24 10 -- 31
.largecircle. 18 0.10 .largecircle. .largecircle. 31 9 -- 13
.largecircle. 19 0.15 .largecircle. .largecircle. 29 10 -- 32
.largecircle. 20 0.31 .largecircle. .largecircle. 24 8 -- 49
.largecircle. 21 0.38 .largecircle. .largecircle. 27 8 -- 58
.largecircle. 22 0.47 .largecircle. .largecircle. 25 8 -- 74
.largecircle. 23 0.60 .largecircle. .largecircle. 32 0 -- 100 X
As shown in Table 1, Samples No. 1 to No. 7 were satisfactory in
all evaluations (coatability, adhesion, internal resistance,
weldability, and coverage) and it was confirmed that carbon black
(conductive material) was disposed in the island shape by the
observation using SEM. The SEM (scanning electron microscope)
picture shown in FIG. 5 is one obtained by photographing the
surface of Sample No. 1 and, in FIG. 5, one division shown in the
lower right part is 10 .mu.m.
Sample No. 8 is a comparative example, which is a case where a
battery cell is prepared using an electrode in which a collector of
only the Al foil as a substrate is used without using carbon black
(conductive material). The internal resistance of the battery cell
was 45 .OMEGA..
Sample No. 9 is the case where the concentration of carbon black
(conductive material) in the slurry is so high as 14% by mass.
Therefore, the coverage with carbon black is so high as 100% and no
island structure was present, so that weldability was not
excellent.
Sample No. 10 is a result of the case where only the water-soluble
resin was used without using the hydrophobic resin. The coverage
was 36%, which fallen within the range of 1 to 80%, and the
coatability, the effect of reducing the internal resistance, and
the weldability were satisfactory. However, sufficient adhesion was
not obtained since the hydrophobic resin was not used.
Sample No. 11 is a result of the case where only the hydrophobic
resin was used without using the water-soluble resin. Since the
water-soluble resin was not used, it was impossible to apply the
slurry containing carbon black on the surface of the substrate
satisfactorily.
As shown in Table 2, Samples No. 12 to No. 22 were satisfactory in
all evaluations (coatability, adhesion, internal resistance,
weldability (1), and coverage). When the attached amount of the
conductive material is 0.01 g/m.sup.2 or more, the internal
resistance of the batteries becomes 90% or less of the internal
resistance (45.OMEGA.) in the case where an Al foil having no
conductive material was used and it could be confirmed that the
contact resistance was significantly decreased.
Sample No. 23 is the case where the concentration of carbon black
(conductive material) in the slurry is so high as 10% by mass.
Therefore, the attached amount of the conductive material on the
substrate surface was large and thus no island structure was
formed, so that the weldability (1) was not excellent.
Table 3 shows a list of characteristic evaluation results and
judgment results of samples prepared using the Cu foil as a
substrate. In Table 3, ".largecircle." represents a good judgment
result and "x" represents a bad judgment result. Moreover, in Table
3, a numerical value judged as bad was underlined.
TABLE-US-00003 TABLE 3 Concentration Concentration Concentration of
of water- Total of Species of hydrophobic soluble Concentration
Sample carbon black hydrophobic resin resin of resins No. Substrate
(% by mass) resin (% by mass) (% by mass) (% by mass) 24 Cu 1 PP 1
1 2 25 Cu 1 PE 0.3 0.3 0.6 26 Cu 5 PP 0.25 1 1.25 27 Cu 2 PP 5 5 10
28 Cu 6 PP 1 1 2 29 Cu 0 -- 0 0 0 30 Cu 10 PP 5 5 10 31 Cu 2 PP 1 0
1 32 Cu 3 PP 0 1 1 Contact Weldability Weldability Sample
resistance (1) (2) Coverage No. Coatability Adhesion (m.OMEGA.
cm.sup.2) (sheet) (sheet) (%) Judgment 24 .largecircle.
.largecircle. 49 -- 10 28 .largecircle. 25 .largecircle. .DELTA. 82
-- 10 8 .largecircle. 26 .largecircle. .DELTA. 21 -- 10 63
.largecircle. 27 .largecircle. .largecircle. 32 -- 10 44
.largecircle. 28 .largecircle. .largecircle. 28 -- 8 50
.largecircle. 29 -- -- 108 -- 10 0 X 30 .largecircle. .largecircle.
14 -- 0 100 X 31 X -- -- -- -- -- X 32 .largecircle. X 41 -- 10 45
X
As shown in Table 3, Samples No. 24 to No. 28 were satisfactory in
all evaluations (coatability, adhesion, internal resistance,
weldability (2), and coverage) and it was confirmed that carbon
black (conductive material) was disposed in the island shape by the
observation using SEM.
Sample No. 29 is a comparative example, which is the case where a
battery cell was prepared using an electrode in which a collector
of only the Cu foil as a substrate was used without using carbon
black (conductive material). The contact resistance was 108
m.OMEGA.cm.sup.2.
Sample No. 30 is the case where the concentration of carbon black
(conductive material) in the slurry is so high as 10% by mass.
Therefore, the coverage with carbon black was so high as 100% and
no island structure was present, so that the weldability (2) was
not excellent.
Sample No. 31 is a result of the case where only the hydrophobic
resin was used without using the water-soluble resin. Since the
water-soluble resin was not used, it was impossible to apply the
slurry containing carbon black on the surface of the substrate
satisfactorily.
Sample No. 32 is a result of the case where only the hydrophilic
resin was used without using the hydrophobic resin. The coverage
was 45%, which fallen within the range of 1 to 80%, and the
coatability, the effect of reducing the internal resistance, and
the weldability (2) were satisfactory. However, sufficient adhesion
was not obtained since the hydrophobic resin was not used.
While the present embodiments of the present invention has been
described in detail and with reference to specific embodiments
thereof, it will be appreciated that the present invention is not
limited thereto, and the present invention will be carried out with
various modification without departing from the scope of the
claims.
This application is based on Japanese Patent Application No.
2011-234320 filed on Oct. 25, 2011 and Japanese Patent Application
No. 2012-194663 filed on Sep. 5, 2012, the entire subject matters
of which are incorporated herein by reference.
INDUSTRIAL APPLICABILITY
According to the electrode material of the present invention, a
conductive material is disposed in the island shape on the surface
of a metal foil that is a substrate and the attached amount of the
conductive material is limited, and thus, the contact resistance
between the active material layer and the electrode material can be
reduced while weldability of a tab or the like is secured.
Moreover, the island structure is adhered to the metal foil that is
a substrate using the hydrophobic resin and water-soluble resin as
binders and thus peeling of the formed island structure can be
suppressed during the handling of the electrode material and in the
manufacturing step before coating with the active material layer,
and the effect of reducing the contact resistance by the island
structure is not impaired.
According to the method for manufacturing the electrode material of
the present invention, since the island structure of the conductive
material can be formed on the surface of the substrate with good
adhesion, an electrode material having a good weldability of a tab
or the like and capable of reducing the contact resistance with the
active material layer, can be manufactured with stable quality.
Moreover, owing to the use of an aqueous solution, an environmental
laod induced by solvent vapor in the manufacturing step can be
reduced.
Further, according to the method for manufacturing the electrode
material of the present invention, since the carbon is used in an
appropriate concentration as a conductive material, an electrode
material in which a suitable island structure is formed can be
manufactured.
Furthermore, according to the method for manufacturing the
electrode material of the present invention, since the resins are
used in appropriate concentration, an electrode material having
good quality can be manufactured.
According to the electrode of the present invention, the contact
resistance between the electrode material and the active material
layer can be reduced.
Moreover, according to the secondary battery of the present
invention, internal resistance can be reduced.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
1: Collector (electrode material)
1a: Substrate
1b: Conductive material
2: Active material layer
10: Electrode
11: Positive electrode (electrode)
12: Negative electrode (electrode)
13: Separator
14: Electrolytic solution
20: Lithium ion secondary battery (secondary battery)
* * * * *